1. Field of the Invention
This invention relates to supplying power to a circuit and particularly to systems including pumped power supplies.
2. Description of Related Art
System designs are routinely constrained by a limited number of power supply voltages (V.sub.CC). For example, consider a portable computer system powered by a conventional battery having a limited power supply voltage. For proper operation, different components of the system, such as a display, a processor, and memory employ several technologies which require power to be supplied at various operating voltages. Components often require operating voltages of a greater magnitude than the power supply voltage or in other cases involve a voltage of reverse polarity. The design of a system, therefore, includes power conversion circuitry to efficiently develop the required operating voltages. One such power conversion circuit is known as a charge pump.
The demand for highly-efficient and reliable charge pump circuits has increased with the increasing number of applications utilizing battery powered systems such as notebook computers, portable telephones, security devices, battery backed data storage devices, remote controls, instrumentation, and patient monitors, to name a few.
Inefficiencies in conventional charge pumps lead to reduced system capability and lower system performance in both battery and non-battery operated systems. Inefficiency can adversely affect system capabilities causing limited battery life, excess heat generation, and high operating costs. Examples of lower system performance include low speed operation, excessive delays in operation, loss of data, limited communication range, and the inability to operate over wide variations in ambient conditions including ambient light level and temperature.
Product reliability is a product's ability to function within given performance limits, under specified operating conditions over time. "Infant mortality" is the failure of an integrated circuit (IC) early in its life due to manufacturing defects. Limited reliability of a charge pump can affect the reliability of the entire system.
To reduce infant mortality, new batches of IC devices (e.g., charge pumps) are "burned-in" before being shipped to customers. Burn-in is a process designed to accelerate the occurrence of those failures which are commonly at fault for infant mortality. During the burn-in process, the ICs are dynamically stressed at high temperature (e.g., 125.degree. C.) and higher-than-normal voltage (for example, 7 volts for a 5 volt device) in cycles that can last several hours or days. The devices can be tested for functionality before, after, and even during the burn-in cycles. Those devices that fail are eliminated.
Conventional pump circuits are characterized by a two part cycle of operation and low duty cycle. Pump operation includes pumping and resetting. Duty cycle is low when pumping occurs at less than 50% of the cycle. Low duty cycle consequently introduces low frequency components into the output DC voltage provided by the pump circuit. Low frequency components cause interference between portions of a system, intermittent failures, and reduced system reliability. Some systems employing conventional pump circuits include filtering circuits at additional cost, circuits to operate the pump at elevated frequency, or both. Elevated frequency operation in some cases leads to increased system power dissipation with attendant adverse effects.
During normal operation of a charge pump, especially charge pumps providing operating voltages higher than V.sub.CC (boosted voltages), certain internal "high-voltage" nodes in the charge pump circuitry reach voltages (over-voltages) having a magnitude significantly higher than either the power-supply voltage or the produced operating voltage. These over-voltages can reach even higher levels under the higher-than-normal voltages used during burn-in testing. When an IC charge pump is tested during a burn-in cycle, high burn-in over-voltages in combination with high burn-in temperatures can cause oxidation of silicon layers of the IC device and can permanently damage the charge pump.
In addition to constraints on the number of power supply voltages available for system design, there is an increasing demand for reducing the magnitude of the power supply voltage. The demand in diverse applications areas could be met with high efficiency charge pumps that operate from a supply voltage of less than 5 volts.
Such applications include memory systems backed by 3 volt standby supplies, processors and other integrated circuits that require either reverse polarity substrate biasing or boosted voltages outside the range 0 to 3 volts for improved operation. As supply voltage is reduced, further reduction in the size of switching components paves the way for new and more sophisticated applications. Consequently, the need for high efficiency charge pumps is increased because voltages necessary for portions of integrated circuits and other system components are more likely to be outside a smaller range.
In view of the problems described above and related problems that consequently become apparent to those skilled in the applicable arts, the need remains, in methods for supplying power to a circuit and particularly in systems including pumped power supplies, for alternatives to the conventional pump circuit having low efficiency, low duty cycle operation, resistance to over-voltage damage and only practically operable from voltages of 5 volts and above.